The present invention relates to thermoplastic polyolefin (xe2x80x9cTPOxe2x80x9d) blends which include a crystalline or semi-crystalline polyolefin and a multimodal elastomer, preferably of a sequentially polymerized ethylene-xcex1-olefin copolymer which has a multimodal distribution of at least one of molecular weight, density or xcex1-olefin comonomers.
Various TPO blends are molded into lightweight, durable articles which are useful as automobile parts, equipment housings, toys and the like. Often, it is desired to paint such components for aesthetic or functional purposes. When these blends contain combinations of crystalline or semi-crystalline polymers and elastomers, however, the surface of the molded article generally must be treated so that the paint can durably adhere to the article. Paint adhesion is a particular concern in articles molded from TPO blends such as those described in U.S. Pat. Nos. 4,480,065, 4,439,573 and 4,412,016.
One way to obtain good paint adhesion is to treat the surface of the article with an interlayer coating to promote or enhance adhesion. When the article is to be used in environments which include high humidity conditions, or is to be exposed to petroleum fuels or solvents, however, the interlayer coatings can be detrimentally affected with reduced paint adhesion as a result. Thus, the paints will chip or peel away during use of the article. An example of this is the use of a painted molded TPO automobile bumper. Such articles will not be approved for use on automobiles unless the paint retains suitable adhesion properties in the presence of such fluids and moisture.
U.S. Pat. No. 5,498,671 discloses a solution to this problem by utilizing a combination of low and high molecular weight ethylene/propylene/diene monomer (EPDM) rubbers with crystalline or semi-crystalline polyolefins. The resultant TPO blends possess excellent adhesion to paints, with superior resistance to petroleum fluids and moisture. A minor drawback of this system is the use of the low molecular weight EPDM, which is a sticky, viscous liquid at room temperature. Thus, it is necessary to carefully handle this viscous fluid, such as by retaining it in plastic bags, or pumping to efficiently facilitate its introduction into an external mixer which mixes the components together. It would be desirable to retain the good paint adherence of such materials, however, while improving the ease of handling of the components during manufacture of the blend.
The present invention relates to a thermoplastic polyolefin blend which includes a polyolefin component of a substantially crystalline polymer in an amount of about 30 to 98 percent by weight of the blend; and an elastomer of a sequentially polymerized ethylene xcex1-olefin copolymer having a multimodal distribution of at least one of molecular weight, density or xcex1-olefin monomers, and being present in an amount of about 2 to 70 percent by weight of the blend.
The polyolefin is preferably present in an amount of about 40 to 96 and most preferably 50 to 95 percent by weight of the blend, and is a crystalline or semi-crystalline polyethylene polymer, polypropylene polymer, or copolymer of ethylene and a C3 to C10 xcex1-olefin. The multimodal elastomer has an overall Mw/Mn ratio of at least 3, is substantially amorphous, and is preferably present in an amount of about 4 to 60 and more preferably about 5 to 50 percent by weight. Advantageously, a bimodal elastomer is used with the different modes being present in a split of between about 75:25 and 25:75.
In one embodiment, at least two modes are present, having weight average molecular weight modes which differ by at least about 25,000 and preferably by about 50,000, 100,000 or more, with the higher molecular weight of the higher molecular weight mode being no greater than about 350,000. One mode may advantageously have a molecular weight which is a multiple of at least about 1.5 and preferably 5 and 50 times higher than that of the other mode.
In an embodiment, termed a xe2x80x9chigh-lowxe2x80x9d split, one mode has a lower molecular weight of about 30,000 or less and the other mode has a higher molecular weight of at least about 150,000 to provide a non-liquid polymer that can be handled as a solid at room temperature. In another embodiment, called the xe2x80x9chigh-highxe2x80x9d split, one mode has a molecular weight of at least about 50,000 and the other has a molecular weight of at least about 100,000. In the xe2x80x9chigh-highxe2x80x9d split, it is advantageous for one of the molecular weights to be about 75,000 and the other molecular weight to be at least about 150,000.
In another embodiment of the invention, at least two modes having densities which differ by at least about 0.005 grams per cubic centimeter (g/cc) are used. Preferably, one mode has a density of greater than 0.85 g/cc and the other mode has a density of less than about 0.96 g/cc, with the difference between densities of the modes being less than about 0.1, preferably less than about 0.05 and more preferably less than 0.03 g/cc.
In yet another embodiment of the invention, at least two modes containing comonomers which differ in length by at least one carbon atom are used. Preferably, the comonomers of the modes differ in length by at least two carbon atoms, and one of which is propene, butene, hexene or octene.
The blends of the invention may also include at least one additional polymeric component in an amount of between about 1 and 20 percent by weight of total blend. At least two different additional polymeric components may be present, but in a total amount of about 3 and 35 percent by weight of the blend. One suitable polymeric component is a copolymer of ethylene and a C3 to C10 (three to ten carbon atoms) xcex1-olefin or a terpolymer of that copolymer and a diene monomer. Another suitable polymeric component is a copolymer of ethylene and an xcex1-olefin which is made with a Kaminsky or metallocene catalyst.
If desired, the blend may include a filler in an amount of about 1 to 30 percent by weight of the blend. Preferred fillers such as talc, mica, glass, or calcium carbonate can be used. Other conventional additives such as nucleating agents, oils and the like can be included if desired.
The blends can be formed into molded articles having one or more outer surfaces, with at least one of the outer surfaces including a coating thereon for aesthetic or functional purposes, if desired. Although any coating can be used, a two component polyurethane material coating is preferred.